Work completed to date has permitted the representation of a "peer behavior system" which is characterized by a demonstrable developmental coherence within the second year of life, which provides significant information about the development of different patterns of dealing with the environment at age 3 1/2, and which implicates behavior in other domains of early functioning as common manifestations of core attributes that act to regulate the likelihood of different paths of developmental progression. The proposed continued work focuses on achieving a deeper understanding of the "core attributes" inferred from the developmental structure of this peer behavior system. Using an extensive pool of longitudinal, multi-setting assessments of cognitive, exploratory and social behavior, hypotheses about conditions which are theoretically necessary to confirm and/or disconfirm the operations of a postulated process are to be examined through analyses of behavior manifested in selected domains of functioning, conditional on the latent structures whose substantive interpretation is at issue. Of particular interest in these analyses is the precise specification of the ways in which the mother-child relationship--implicated by completed work as one factor which affects differences in manifest modes of reacting to peers--can be understood as contributing to the vicissitudes of developmental processes.